1. A man pushes on the wall of a building with force F.
a) How much work is done by the man?
W=Fd; d=0 so W=0 J
b) How much work per second is done by the man according to a person
moving with a velocity v away from the wall?
W=Fd; d=vt; W=Fvt; so W/t=Fv
c) Does the kinetic energy of the wall increase due to this work? If
not, explain why not.
No it doesn't. Since the net force on the wall
is zero the net work is zero so the change in KE is zero.(Note, in the
frame of the person
moving away from the wall the wall is moving
with velocity -v and F is doing work, but the ground is doing negative
work on the wall to compensate)
2. A brick is resting on a concrete surface. Which of the following
is true? I. the force required to overcome static friction is greater than
the force required to overcome dynamic friction; II. the force required
to overcome dynamic friction is greater than the force required to overcome
static friction; III. friction is proportional to the component of weight
acting normally on the surface
a) all are correct
b) I and II are correct
c) I and III are correct
d) II and III are correct
3 Compared to the amount of energy required to accelerate a car from rest to 10 miles per hour, the amount of energy required to accelerate the same car from 10 mph to 20 mph is
a) the same.
b) twice as much.
c) three times as much.(note energy required
from 0mph to 20mph 4 times as great)
d) four times as much.
4. A puck moving across a horizontal surface experiences constant negative acceleration due to friction, and comes to rest after 12 seconds. How does the energy lost in the first 6 seconds compare to the energy lost in the last 6 seconds?
a) first half > second half (frictional force
is constant, but distance travelled is greater in first 6 seconds than
in the second)
b) first half = second half
c) first half < second half
d) too little information